This invention relates to conductive single-component developers. More particularly, this invention relates to methods for making conductive dry toners which are suitable for developing an electrostatic latent image formed by electrophotography, electrostatic recording, ionography and the like.
Recording systems for visualizing image information through an electrostatic latent image, such as electrophotography, are now widespread in various fields. In electrophotography, an electrophotographic photoreceptor is charged and then exposed to light to form an electrostatic latent image, the latent image is developed with a developer containing a toner, and the toner image is transferred and fixed. Developers used in electrophotography include two-component developers containing a toner and a carrier, and single-component developers containing a toner and no carrier.
Conductive toners for single-component developers used in electrophotography and methods for making them are disclosed, for example, in U.S. Pat. Nos. 3,639,245 (Nelson) and 3,196,032 (Seymour).
According to the method taught in the Nelson patent, a dry-powder blend is first obtained by any of several standard means, for example, by melting a resin, stirring in the solid filler, if any, allowing the mixture to cool, then grinding and classifying to the appropriate particle size range of approximately 1 to 15 microns diameter. This powder, which is pseudocubical in shape, is then "spheroidized" by aspirating the powder into a moving gas stream, preferably air, thus creating an aerosol, and directing the aerosol at about 90.degree. (plus or minus 5.degree.) through a stream of hot air, which has been heated to about 900.degree.-1100.degree. F., into a cooling chamber, where the powder is then allowed to settle by gravity while it cools. The resulting powder is made up of substantially spherical particles. It is then dry blended with conductive powder, such as conductive carbon black, and the mixture is directed at about 90.degree. (plus or minus 5.degree.) through a stream of gas, preferably air, heated to a temperature (e.g., 700.degree.-800.degree. F.) which can at least soften and desirably melt the thermoplastic resin in the particles and maintain that softened or molten condition for a period of time sufficient to permit the conductive powder to become essentially completely embedded onto the resin particle surface, due to the effects of surface tension.
A drawback to the Nelson method is that the high temperature (700.degree.-800.degree. F.) used therein allows for only a brief heat treatment period, several seconds, for the toner/conductive powder mixture. Longer heat treatment periods could cause the toner/conductive powder particles to soften to the point that they would begin to adhere together. The short heat treatment period reduces tolerance for variations in operating temperatures and process times, resulting in poor control of the process. Poor process control in turn limits the variations which can be made to the process to adjust the final properties of the toner.
U.S. Pat. No. 3,196,032 to Seymour discloses a method of making electrostatic ink powder by means of fluid bed processing equipment. In the Seymour method, a dry mixture of resin particles and conductive powder particles is introduced into fluid bed processing equipment, wherein pressurized dry air is passed upwardly through the mixture to form a dense phased fluidized mass. A solvent vapor in which the resin is soluble is passed through the mixture, whereby the resin powder is slightly softened and made relatively tacky so that particles of the conductive powder become partially embedded in and bonded to the surfaces of the resin material. The fluidized mass is then dried with pressurized air without the solvent to a powder consistency. The particle size of the resin powder is preferably 25-50 microns and the particle size of the conductive powder is preferably 8-25 millimicrons.
Disadvantages of the Seymour method include its use of particles which are typically too large for modern toners (the larger the toner particle size, the lower the resolution of the print) and its use of a solvent. Drawbacks to solvent use include higher costs due to the use of an additional ingredient (solvent), environmental hazards commonly associated with solvents, and problems involved in removing solvent after completion of the process.